Power supplies, such as switching power supplies, are used for providing direct current (DC) supply for various electronic devices, such as base stations in a cellular communications system. For a power supply, an electronic device is a load which often includes a capacitive load component. When power with a specified operating voltage is connected to such a load, a high current is initially formed for charging the capacitive load component. Such a high current may exceed the current output rating of the power supply, and this may cause power components of the power supply to get damaged. Another cause for excessively high load current may be a short circuit or other fault in the load. It is also possible that the load does not match with the power supply ratings. In order to protect a power supply in such situations overload protection circuits are used. These circuits limit and/or disconnect the current supplied to the load in situations where excessive current values are detected at the output of the power supply.
FIG. 1 illustrates an overload protection circuit according to prior art. The circuit is connected between the power output line −VSUPPLY of a power supply and power input line −VLOAD of the load. R is used in the Figure to denote resistive part of the load and C is used to denote capacitive part of the load. The circuit has a switching element Q, such as a FET transistor, and a control unit 15 for controlling the gate of the transistor and thus current supply to the load. The control unit monitors the supply current with a current sensor 18. When the transistor is switched-on the supply current flows to the load through the transistor. When the current exceeds a predetermined limit the output 16 of the control unit controls the transistor to reduce the current. The transistor Q is thus also used for linear control of the supply current.
However, this solution of the prior art has a certain disadvantage. transistors cannot dissipate high power for long periods without exceeding safe operating area (SOA) of the transistor. If the SOA of the transistor is exceeded there is a risk of the transistor becoming damaged. The reliability of the device may thus be degraded. As a further disadvantage, providing linear control of the switching transistor requires a more complicated structure of the control circuit. There is also a potential risk of instability related to such a linear control.
FIG. 2 illustrates another overload protection circuit according to prior art. This circuit has a first switching element Q1 between the power supply and the load. The overload protection circuit also has a second switching element Q2, which is connected in series with a power resistor R1. The switching elements are controlled by a control circuit 25. When the power is switched ON to the load the second switching element is first switched ON by the control line 27, allowing the current to flow through the second switching element Q2 and the power resistor R1. The current is limited by the resistor in order to avoid an excessive value of current. After the load capacitance has been initially charged the first switching element is switched ON by the control line 26, allowing the current to flow directly through the first switching element to the load. There may be, for example, a fixed delay arranged between switching ON the second and first switching elements.
There are certain disadvantages also concerning the prior art overload protection circuit according to FIG. 2. Firstly, the load voltage does not reach the nominal value when the current is supplied through the resistor R1. Therefore it is possible that a high charging current still exists when the first switching transistor is switched ON.
Secondly, the power resistor may dissipate a large amount of energy during the power start-up. Therefore, the power resistor must have a high power rating. Such a power resistor has large dimensions and thus requires much space. It is also a relatively expensive component, thus increasing the production cost of the device. The required resistance and power ratings of the power resistor also depend on the requirements of the power supply and load. Therefore, it may be necessary to provide several versions of overload protection circuits with different components.
The circuit of FIG. 2 also requires two power switching elements and corresponding control circuits for both elements. This further increases the complexity and production cost of the device.
In electronic systems, such as communication systems, it is often necessary to supply power of various voltages to several devices of the system. An overload protection circuit is needed for each supply connection, and the required number of the overload protection circuits may therefore be high. It is often also necessary to have different properties of overload protection circuits for different load devices and different supply inputs of each device. Further, it is important that the reliability of the overload protection circuits is high because a failure may cause large part of an electronic system to become inoperative. As described above, the overload protection circuits according to the prior art do not fulfil these requirements in a desirable manner.